Breaking changes
- Require clients to make explicit when input variables cover a range of dates, rather than allowing inputs to be derived from past or future values; also removes the `missing_value` base function
Migration notes
You might need to change your code if any of the following applies:
- you are a model author, and there are variables in your model that use a `base_function` attribute
- your model **or application** uses any non-numeric variables (boolean, date, string or Enum) used in, for which the period on which you define **inputs** does not match the period for which you are requesting **outputs**
Detailed instructions are provided in the [PR description](https://github.com/openfisca/openfisca-core/pull/813).